In the art of chemical pulping, natural cellulose material, for example, softwood chips, is treated to produce cellulose pulp from which paper products are made. As a prerequisite to this treatment, the cellulose material is typically introduced and conditioned prior to being formally "cooked" in pressurized vessels, that is digesters, by what is known in the art as a "feed system". Since their introduction in the 1940s and 1950s, feed systems for continuous digesters have been essentially unchanged. However, U.S. Pat. No. 5,476,572 introduced the first significant development to the means of feeding a chip slurry to a digester, either continuous or batch, since the initial development of chip feeding systems. The system disclosed in the U.S. Pat. No. 5,476,572 and marketed under the name LO-LEVEL.RTM. Feed System by Ahlstrom Machinery Inc. of Glens Falls, N.Y., first introduced the concept of pumping a slurry of chips and liquor into a high-pressure transfer device instead of using a downstream pump to draw the slurry into the transfer device. As described in the '572 patent (the disclosure of which is incorporated by reference herein) this system dramatically reduces the complexity of the overall feed system by permitting, among other things, the high pressure transfer device to be positioned at, for example, ground level instead of elevated as was required by the prior art. Further improvements to the system initially disclosed in the '572 patent are described in U.S. Pat. Nos. 5,622,598 and 5,635,025 (the disclosures of which are also incorporated by reference herein).
As disclosed in co-pending application Ser. No. 08/744,857, filed on Nov. 4, 1996 and Ser. No. 08/738,239, filed on Oct. 25, 1996, the ability to pump a slurry of chips provides numerous opportunities to improve the efficiency by which comminuted cellulosic material can be introduced to a cooking system. The present invention provides further improvements to the feeding system for the chemical treatment of wood, particularly wood chips. For example, one embodiment of the present invention comprises a refinement of the invention disclosed in application Ser. No. 08/744,857 (the disclosure of which is incorporated by reference herein). Specifically, one embodiment of this invention comprising the refinement of the system illustrated in FIG. 2 of application Ser. No. 08/744,857, wherein instead of splitting the flow path into two or more paths to distinct digesters, two or more flow paths are used to feed a single digester. This is particularly advantageous when the capacity of one component of the feed system is exceeded by the desired capacity of the entire pulping system, or if the cost of manufacturing a larger capacity device is either technically or economically unfeasible.
The present invention also addresses the problem of isolating and removing undesirable material from the fiberline to avoid interference with the process or damage to the equipment. The comminuted cellulosic fibrous material, for example, softwood chips, that are treated in conventional pulping systems typically contains non-cellulose debris, for example, sand, dirt, stones, miscellaneous metal parts (for example, nails, pieces of wire, nuts and bolts) or metal fragments, or other heavy cellulose (e.g. knots) or non-cellulose material. This material is collectively referred to as "tramp material", and typically has a density at least about 10% greater than the cellulose material being processed (e.g. at least 50% greater). Much of this material is separated during chip preparation, but some still passes to the digester feed system and to the digester itself. Conventionally, this material can be separated from the chips in the feed system by some form of separator. for example, a Tramp Material Separator marketed by Ahlstrom Machinery Inc., of Glens Falls, N.Y. One such Separator is shown schematically as item 12 in U.S. Pat. No. 4,743,338. This Separator is described in the brochure entitle "Digester Update", 4.sup.th Edition, published in September 1981 by Kamyr, Inc. (now Ahlstrom Machinery Inc.) Tramp material may also be separated from the fiberline downstream of the digester, after the chips have been converted to a slurry of fibers and liquid. For example, the MC.RTM. Tramp Material Separator described in a 1986 pamphlet published by Kamyr, Inc., marketed by Ahlstrom Machinery Inc., and illustrated in U.S. Pat. No. 4,737,274, may be located in the blowline of a digester wherever convenient. Tramp material may also be separated from a liquid stream. U.S. Pat. No. 4,280,902 illustrates a cyclone-type separating device for removing undesirable material, in particular sand and the like, from a liquid stream in the feed system. This device is marketed under the name Sand Separator by Ahlstrom Machinery Inc. Though these devices have proven to be effective in removing tramp material from the feed systems of digesters, the introduction of the Lo-Level.RTM. feed system provides additional novel methods for isolating and removing such undesirable material.
According to one aspect of the present invention a tramp material separator for use in a comminuted cellulosic fibrous slurry feed system, e.g. for a digester, is provided. The separator comprising the following components: A first conduit having a top portion including an inlet and a bottom portion below the top portion, and an outlet. Means for providing centrifugal force on a slurry flowing in the first conduit to cause less dense solids in the slurry to move in a first flow path, and more dense, tramp material, solids in the slurry to separate from the first flow path and move in a second flow path under the influence of centrifugal force; the means for providing centrifugal force consisting essentially of a radiused section of the first conduit adjacent the bottom portion thereof, so that no moving or powered elements are provided for effecting separation. And a cavity defined adjacent and below the radiused section of the first conduit for receipt of more dense solids flowing in the second flow path.
This system may be used to feed comminuted cellulosic fibrous material to a digester, continuous or batch, or it may be used in any system that transfers comminuted cellulosic fibrous material that contains tramp material that is preferably separated and removed. For example, this system may be used in a chip transport system as disclosed in copending application Ser. No. 08/738,239 filed on Oct. 25, 1997 (the disclosure of which is included here by reference).
The separator may further comprise a baffle adjacent a portion of the cavity most downstream of the cavity in the first flow path, the baffle extending into the first flow path to aid in directing more dense, tramp material, solids into the cavity and retaining the tramp material in the cavity. Also the tramp metal separator preferably further comprises a nozzle for introducing liquid into the top portion of the first conduit at high speed so as to maximize the flow rate of slurry in the first flow path, and thereby enhance the centrifugal force moving more dense, tramp material, solids in the second path.
The separator may further comprise means for intermittently removing tramp material from the cavity, or for continuously removing it. The intermittent removal means may comprise any conventional device for removing trapped material. Preferably the means for intermittently removing tramp material from the cavity comprises a first valve closest to the cavity, a second valve remote from the cavity, and a chamber between the first and second valves, the first and second valves independently operable (although a conventional system/interlock is used to see that they are not both open at the same time) to allow tramp material to collect in the chamber when the first valve is open and the second valve is closed, and to allow discharge of tramp material from the chamber when the second valve is at least partially opened and the first valve is at least partially closed.
The separator also preferably comprises means for establishing a purged flow of fluid into the cavity for effecting movement of less dense solids (the cellulose material itself that flow into the cavity back into the first flow path. The purge flow establishing means may comprise any suitable conventional conduit, nozzle, deflector, valve, baffle, or the like that secures the desired purge flow.
The first conduit may be substantially circular in cross section (although it might also be rectangular or have other configurations), and may have a first diameter at the top portion thereof and a transition to a second diameter larger than the first diameter at the bottom portion thereof before the outlet. The first conduit, including the radius section thereof, may make substantially a 90.degree. angle from the inlet to the outlet, the outlet being substantially horizontal and the inlet substantially vertical.
Note that the separation of tramp material from chip or fiber slurries according to the invention is different from the separation of undesirable or oversized material from low or medium consistency pulp streams. These processes which are typically referred to as "cleaning" or "screening", typically separate much smaller debris or uncooked wood material from the pulp stream. The present invention is particularly applicable to the separation of tramp material from a slurry of cellulose chips and liquid in the feed system of a digester, either continuous or batch.
Another embodiment of the present invention comprises an improvement of the feed system described U.S. Pat. No. 5,622,598 and in copending application Ser. No. 08/744,857 to remove tramp material from the feed system. This embodiment includes a conveyor for feeding comminuted cellulosic fibrous material including at least some tramp material, comprising: a housing having a first end and a second end; an inlet located adjacent said first end; an outlet adjacent said second end; a screw conveyor extending from said first end to said second end for conveying said material from the inlet to the outlet; a cavity located beneath the conveyor for collecting tramp material; a liquor inlet in the cavity for introducing liquid to the cavity so that the liquid agitates and conveys the desirable fibrous material from the cavity to the outlet while allowing the undesirable tramp material to collect in the cavity; and means for removing the collected tramp material from the cavity.
That is, according to this aspect of the present invention a chemical cellulose pulp digester feed system is provided comprising (as conventional components) a chip bin, a metering device, a conduit for entraining comminuted cellulosic material from the metering device in cooking liquor to provide a slurry, and a transfer device for pressurizing the slurry for feeding it to a digester; and according to the present invention, the metering device comprises a substantially horizontal axis metering screw having a housing with an inlet, an outlet, a rotating screw extending between the inlet and the outlet inside the housing, and a tramp material separator between the inlet and the outlet. Preferably the tramp material separator comprises a cavity adjacent the outlet, and extending downwardly from the screw housing so that more dense, tramp material, solids will flow into the cavity due to density differences between the tramp material and the slurry, and as a result of the rotating screw moving the more dense tramp material outwardly toward the housing. The system preferably further comprises means for establishing a purge flow of fluid into the cavity for effecting movement of less dense solids that flow into the cavity back out of the cavity while allowing more dense tramp material to flow into the cavity. There may also further be means for intermittently removing the tramp material from the cavity, as described above.
Another embodiment of this invention comprises an apparatus for treating comminuted cellulosic fibrous material including at least some tramp material, comprising: a cylindrical treatment vessel (e.g. chip bin) fed with comminuted cellulosic fibrous material; a metering device operatively connected to the treatment vessel; a conduit operatively connected to the metering device and having means for isolating said tramp material from the comminuted cellulosic fibrous material; and a pump operatively connected to the conduit having an outlet operatively connected to at least one digester. The treatment vessel is preferably a steaming vessel in which the comminuted cellulosic fibrous material is exposed to steam. Furthermore, this vessel is preferably a Diamondback.RTM. steaming vessel sold by Ahlstrom Machinery Inc. and described in U.S. Pat. No. 5,500,083. The means for feeding material to the steaming vessel may be any form of device which can introduce comminuted cellulosic fibrous material to a vessel but is preferably one that minimizes or prevents the escape of gases while material is being introduced, such as a screw-type conveyor having a hinged gate as disclosed in co-pending application Ser. No. 08/713,431 filed on Sep. 13, 1996 (the disclosure of which is incorporated by reference herein).
The metering device may be any form of suitable metering device, such as a Chip Meter as sold by Ahlstrom Machinery Inc., but is preferably a screw-type metering device as disclosed in U.S. Pat. No. 5,622,598, having one or more parallel screws. The conduit may be any form of pipe, chute, or tube for conveying the chips by means of gravity from the metering device, but is preferably a tube having a radius of curvature as shown in co-pending application Ser. No. 08/738,239, or a radiused elbow.
The means for isolating the tramp material preferably comprises or consists of a cavity or "trap" located in the metering device or in the conduit leading from the metering device to the pump, and as described above.
According to another aspect of the present invention, a chemical cellulose pulp digester feed system is provided comprising the following conventional components: a chip bin, a metering device, a conduit for entraining comminuted cellulosic material from the metering device in cooking liquor to provide a slurry, a tramp material separator, and a transfer device for pressurizing the slurry for feeding it to a digester. According to the invention the transfer device comprises a slurry pump for feeding slurry to a feeder, and the tramp material separator comprises a cyclone separator between the slurry pump and the feeder. The feed system further preferably comprises a plurality of the cyclones connected between the slurry pump and the feeder, either in series or in parallel, and optionally connected to the plurality of feeders.
According to another aspect of the present invention, a method of separating tramp material from a slurry of cellulosic fibrous material in a liquid having a solids consistency of at least 5% (preferably the conventional solids consistency for feeding a slurry of comminuted cellulosic fibrous material to a continuous or batch digester, typically about 10-15%). In this context, it is to be understood that a solids consistency of, for example, 5%, refers to the weight percent of the non-dissolved solids, for example the wood chips, in the slurry. Liquid streams in and around pulp mills often contain dissolved solid material, the content of which is typically expressed as a percent. The method comprises the following steps: (a) causing the slurry to flow in a generally downward flow in a first flow path. (b) Without impacting the slurry with a rotating or reciprocating mechanical member, causing the first flow path to bend smoothly and sharply toward the horizontal, so as to provide a centrifugal force on the slurry to cause less dense solids in the slurry to continue to move in a first flow path, and more dense, tramp material, solids in the slurry to separate from the first flow path and move in a substantially downward second flow path under the influence of centrifugal force into a cavity below the first flow path. And (c) removing the separated tramp material from the cavity.
Step (b) may be further practiced by introducing liquid under high speed into the slurry so as to maximize the flow rate of slurry in the first flow path, and thereby enhance the centrifugal force moving more dense, tramp material, solids in the second path. There may also be the further step of introducing a purge flow of fluid into the cavity for effecting movement of less dense solids that flow into the cavity back into the first flow path. There also may be the further step of placing a baffle adjacent a portion of the cavity most downstream of the cavity in the first flow path so that the baffle extends into the first flow path to aid in directing more dense, tramp material, solids into the cavity and retaining the tramp material in the cavity. The apparatus for practicing the method is preferably as described above.
It is a primary object of the present invention to provide an effective method and system for feeding a chemical pulp digester, and particularly tramp material separating structures and methods associated therewith. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.